The present invention is generally directed to toner compositions, developers thereof, and toner preparative processes, and more specifically, to a preparative process which involves aggregation of latex, colorant, and additive particles into toner sized aggregates, followed by coalescence or fusion of the latex particles within the aggregates to form integral toner particles to provide toner compositions. In embodiments, the present invention is directed to a chemical in situ preparative process for toners without the need to utilize conventional pulverization and classification methods, thus rendering the present process economical and wherein toner compositions with a particle size as herein defined by volume average diameter of from about 1 to about 20, and preferably from 2 to about 10 microns, and narrow particle size distribution as conventionally characterized by GSD (geometric standard deviation) of, for example, from about 1.10 to about 1.35, and more specifically, from about 1.15 to about 1.25 as measured on the Coulter Counter can be obtained. The resulting toners can be selected for known electrophotographic imaging and printing processes. In embodiments, the present invention is directed to toners based on addition polymer resins derived from emulsion polymerization of a mixture of styrene, isoprene, acrylate or methacrylate, and acrylic acid monomers, and a preparative process thereof comprised of blending by high shearing device a latex emulsion stabilized with an ionic surfactant, and an optional nonionic surfactant with an aqueous pigment dispersion containing an oppositely charged ionic surfactant and optional charge control additive, and other known toner additives. The volume average diameter of the latex particles suitable for the process of the present invention is from about 0.01 micron to about 1.0 micron, and preferably from about 0.05 to about 0.5 micron, while the amount of each ionic surfactant ranges from about 0.01 percent to about 10 percent by weight of the total amount of the reaction mixture. The mixing of the two oppositely charged surfactants induces flocculation of latex, pigment, surfactants, and optional additive particles, which flocculent mixture, on heating with gentle mechanical stirring at a temperature range of from about 25.degree. C. below to about 1 .degree. C. below the glass transition temperature (Tg) of the latex resin enables the formation of electrostatically bound toner sized aggregates comprised of latex, pigment, and optional additive particles. The size of the aggregates is primarily dependent on the temperature at which aggregation is carried out, and for a given latex composition, larger aggregates are obtained at higher temperatures, provided that the temperature is not above the Tg of the resin so as to cause fusion or coalescence of the latex particles. The particle size distribution of the aggregates does not appear to be dependent on the aggregation temperature, and is generally narrow as typified by a GSD of less than 1.35, and more specifically, of less than about 1.25. These aggregates, which have a volume average diameter of from about 1 to about 20 microns, are then subjected to further heating in the presence of additional anionic surfactant at a temperature above the Tg of the latex resin, and more specifically, at a temperature ranging from about 10.degree. C. to 50.degree. C. above the Tg for a duration of 30 minutes to a few hours to effect fusion or coalescence of the latex particles within the aggregates to form integral toner particles. The degree of coalescence is dependent on the temperature and duration of the heating. Suitable temperatures for coalescence range, for example, from slightly above the Tg to over 100.degree. C., depending on the nature of the latex resin, its composition, the pigment and optional additives. In general, the coalescence is conducted at a temperature of between about 65.degree. C. to about 110.degree. C., and preferably between about 75.degree. C. to about 105.degree. C. The resulting toner particles retain the size of the precursor aggregates, that is the volume average particle size of the aggregates is substantially preserved during coalescence wherein electrostatically bound aggregates are converted to integral toner particles as a result of the fusion of the latex particles within the aggregates. In another embodiment thereof, the present invention is directed to an economical chemical process comprised of first blending by high shear mixing an aqueous pigment dispersion containing a pigment, such as HELIOGEN BLUE.TM. or HOSTAPERM PINK.TM., and a cationic surfactant, such as benzalkonium chloride (SANIZOL B-50.TM.), with a latex emulsion comprised of suspended relatively low molecular weight latex resin particles derived from emulsion polymerization of styrene, isoprene, acrylate or methacrylate, and acrylic acid monomers. The latex emulsion is generally stabilized with an anionic surfactant, such as sodium dodecylbenzene sulfonate, for example NEOGEN R.TM. or NEOGEN SC.TM., and a nonionic surfactant, such as alkyl phenoxy poly(ethylenoxy)ethanol, for example IGEPAL 897.TM. or ANTAROX 897.TM.. The latex size ranges from, for example, about 0.01 to about 1.0 micron in volume average diameter as measured by the Brookhaven Nanosizer. The mixing of the two dispersions with two oppositely charged surfactants induces flocculation of the latex, pigment, optional additive particles and surfactants, which flocculent mixture on heating at a temperature of from about 25.degree. C. to about 1.degree. C. below the Tg of the latex resin results in the formation of electrostatically bound aggregates ranging in size from about 2 microns to about 10 microns in volume average diameter as measured by the Coulter Counter. On subsequent heating at about 10.degree. C. to about 50.degree. C. above the Tg of the resin in the presence of additional anionic surfactant, the aggregates are converted into integral toner particles. The aforementioned toners are especially useful for the development of colored images with excellent image resolution, color fidelity, and image projection efficiency.
While not being desired to be limited by theory, it is believed that the aggregation is caused by the attraction between or neutralization of two oppositely charged surfactants, one absorbed on the pigment and optional additive particles, and the other on the latex particles. The aggregation process is temperature dependent, and is faster at higher temperatures. Subsequent heating of the aggregates at a temperature of, for example, 10.degree. C. to 50.degree. C. above the latex resin Tg fuses or coalesces the latex particles within the aggregates, enabling the formation of integral toner particles comprised of polymer resin, pigment particles, and optionally charge control agents. Furthermore, in other embodiments the ionic surfactants on the pigment and latex particles can be interchanged, such that the pigment dispersion contains an anionic surfactant, while the latex emulsion contains a cationic surfactant. It is of importance in the processes of the present invention in embodiments that proper temperature control be exercised as the temperature affects both the aggregate size during aggregation, and the shape and surface morphology of the resulting toner particles during coalescence or fusion. Similarly, to obtain toners of the present invention with the required performance characteristics, critical selection of certain latex compositions derived from emulsion polymerization of styrene, isoprene, acrylate or methacrylate, and acrylic acid monomers is mandatory.
In U.S. Pat. No. 5,366,841, the disclosure of which is totally incorporated herein by reference, there are illustrated emulsion/aggregation processes, and more specifically, a process for the preparation of toner compositions comprising:
(i) preparing a pigment dispersion in water, which dispersion is comprised of a pigment, an ionic surfactant and optionally a charge control agent;
(ii) shearing the pigment dispersion with a latex blend comprised of resin particles, an ionic surfactant of opposite charge polarity to that of said ionic surfactant in the pigment dispersion and a nonionic surfactant thereby causing a flocculation of resin, pigment, and charge control additive particles to form a uniform dispersion of solids in the water, and surfactant;
(iii) heating the above sheared blend at a critical temperature region about equal to or above the glass transition temperature (Tg) of the resin, while continuously stirring to form electrostatically bounded toner size aggregates with a narrow particle size distribution, and wherein said critical temperature is from about 0.degree. C. to about 10.degree. C. above the resin Tg, and wherein the resin Tg is from about 30.degree. C. to about 65.degree. C. and preferably in the range of from about 45.degree. C. to about 65.degree. C.;
(iv) heating the statically bound aggregated particles from about 10.degree. C. to about 45.degree. C. above the Tg of the resin particles to provide a toner composition comprised of polymeric resin, pigment and optionally a charge control agent; and
(v) optionally separating and drying said toner.
As examples of resins, in the U.S. Pat. No. 5,366,871 patent is indicated that there may be selected polymers selected from the group consisting of poly(styrene-butadiene), poly(para-methyl styrene-butadiene), poly(meta-methyl styrene-butadiene), poly(alpha-methyl styrene-butadiene), poly(methylmethacrylate-butadiene), poly(ethylmethacrylate-butadiene), poly(propylmethacrylate-butadiene), poly(butylmethacrylate-butadiene), poly(methylacrylate-butadiene), poly(ethylacrylate-butadiene), poly(propylacrylate-butadiene), poly(butylacrylate-butadiene), poly(styrene-isoprene), poly(para-methyl styrene-isoprene), poly(meta-methyl styrene-isoprene), poly(alpha-methylstyrene-isoprene), poly(methylmethacrylate-isoprene), poly(ethylmethacrylate-isoprene), poly(propylmethacrylate-isoprene), poly(butylmethacrylate-isoprene), poly(methylacrylate-isoprene), poly(ethylacrylate-isoprene), poly(propylacrylate-isoprene), and poly(butylacrylate-isoprene); terpolymers, such as poly(styrene-butadiene-acrylic acid), poly(styrene-butadiene-methacrylic acid), PLIOTONE.TM. available from Goodyear, polyethylene-terephthalate, polypropylene-terephthalate, polybutylene-terephthalate, polypentylene-terephthalate, polyhexalene-terephthalate, polyheptadene-terephthalate, polyoctalene-terephthalate, and the like. With the present invention, there are provided toners based on certain styrene-isoprene-acrylate-acrylic acid or styrene-isoprene-methacrylate-acrylic acid resin derived from 70 to 85 weight percent of styrene, 5 to 20 weight percent of isoprene, 1 to 15 weight percent of acrylate or methacrylate, and 0.5 to 5 weight percent of acrylic acid; the weight average molecular weight (M.sub.w) of the resin relative to the styrene standards is from about 20,000 to about 40,000 while the number average molecular weight (M.sub.n) is from about 5,000 to about 10,000. Advantages achievable with the toners of the present invention include, for example, lower toner fusing temperatures of from about 135.degree. C. to about 170.degree. C., enhanced image resolution from narrow toner particle size distribution, low or no image background noise from narrow toner triboelectric charge distribution and lesser extent of out-of-specification fine particles, high image gloss and excellent image fix characteristics enabled by the relatively low molecular weight resin of specific compositions derived from emulsion polymerization of styrene, isoprene, acrylate or methacrylate, and acrylic acid monomers in embodiments of the present invention. All these attributes have contributed to the attainment of high image quality.
There is illustrated in U.S. Pat. No. 4,996,127 a toner of associated particles of secondary particles comprising primary particles of a polymer having acidic or basic polar groups, and a coloring agent. The polymers selected for the toners of the '127 patent can be prepared by an emulsion polymerization method, see for example columns 4 and 5 of this patent. In column 7 of this '127 patent, it is indicated that the toner can be prepared by mixing the required amount of coloring agent and optional charge additive with an emulsion of the polymer having an acidic or basic polar group obtained by emulsion polymerization. Also, in column 9, lines 50 to 55, it is indicated that a polar monomer, such as acrylic acid, in the emulsion resin is necessary, and toner preparation is not obtained without the use, for example, of acrylic acid polar group, see Comparative Example I. Additionally, the process of the '127 patent does not appear to utilize counterionic surfactant and flocculation process as does the present invention, and does not use a counterionic surfactant for dispersing the pigment. In U.S. Pat. No. 4,983,488, there is illustrated a process for the preparation of toners by the polymerization of a polymerizable monomer dispersed by emulsification in the presence of a colorant and/or a magnetic powder to prepare a principal resin component and then effecting coagulation of the resulting polymerization liquid in such a manner that the particles in the liquid after coagulation have diameters suitable for a toner. It is indicated in column 9 of this patent that coagulated particles of 1 to 100, and particularly 3 to 70 are obtained. This process is thus directed to the use of coagulants, such as inorganic magnesium sulfate, which results in the formation of particles with wide GSD. In U.S. Pat. No. 4,797,339, there is disclosed a process for the preparation of toners by resin emulsion polymerization, wherein similar to the '127 patent polar resins of opposite charges are selected, and wherein flocculation, as in the present invention, is not disclosed; and in U.S. Pat. No. 4,558,108, there is disclosed a process for the preparation of a copolymer of styrene and butadiene by specific suspension polymerization. Other prior art that may be of interest includes U.S. Pat. Nos. 3,674,736; 4,137,188 and 5,066,560.
The process described in the present application has several advantages as indicated herein including the effective preparation of small toner particles with narrow particle size distribution without the need to utilize conventional classification processes; the process is very energy efficient as it is a wet process and does not involve energy intensive grinding or pulverization, and classification processes, high process and materials yields, short or reduced process times, and shorter or reduced change over time for preparing different color toners, therefore rendering it attractive and economical. The process of the present invention is particularly efficient for generating particle size below 10 microns, or more specifically, below 8 microns, which is in the regime where conventional pulverization methods become very cost ineffective.